Radial turbine rotors used in gas turbine engines are subjected to very high temperatures, severe thermal gradients, and very high centrifugal forces. The turbine blades are located directly in and are directly exposed to the hot gas-stream. The inducer tips of the blades therefore experience the highest temperatures and consequently are most susceptible to creep rupture failure that could result in an inducer tip striking the surrounding nozzle enclosure, causing destruction of the turbine. The turbine hub is subjected to very high radial tensile forces and also is susceptible to low-cycle fatigue damage. In order to achieve optimum blade and hub material properties, dual alloy structures have been used in which the hub is formed of wrought superalloy material having high tensile strength and high low-cycle fatigue strength, while the blade ring, including the blades (i.e., air foils) and blade rim, is formed of superalloy material having high creep rupture strength at very high temperatures. The dual alloy approach has been used where very high performance turbine rotors are required, because in very high performance turbine rotors, materials that have optimum properties for the turbine blades do not have sufficiently high tensile strength and sufficiently high low-cycle fatigue strength for use in the turbine hubs.
U.S. Pat. No. 4,335,997 by Ewing et al. discloses a dual alloy radial turbine rotor in which a preformed hub of powdered metal is consolidated into a preform having a cylindrical nose section and an outwardly flared conical skirt. After machining, the outer surface of the hub is diffusion bonded (by hot isostatic pressing) to a cast blade ring. The slope of a flared skirt portion of the blade ring is configured to optimize the location of the high strength material and achieve optimum blade and hub stress levels.
Although not recognized by the Ewing et al. reference, a problem that occurs in radial turbine rotors, is the occurrence of cracking in the "saddle" regions of the rim of the blade ring. Our analyses and experiments have shown that high creep rupture strength material of which the blade ring is formed does not adequately resist fatigue in the saddle regions at the outer portions of the conical skirt of the rim of the blade ring.
The blades in the Ewing et al. reference have cooling passages therein, resulting in a considerably lower temperature profile than would be the case for a non-cooled blade structure. Therefore, the creep rupture strength of the blade material could be lower for the Ewing et al. blade structure than for a non-cooled blade structure in the same environment. However, cooled blades are much more expensive to manufacture than non-cooled blades. It would be desirable to provide a non-cooled blade having a grain structure or morphology that can withstand failure due to creep rupture. It is also desirable that a non-cooled blade structure be provided in a radial turbine rotor that is resistant to fatigue and cracking in the saddle regions between the blades.
Numerous prior art references disclose axial dual alloy turbine wheels, but none of them are subjected to the hot radial gas flow patterns that result in cracking in the saddle regions of radial turbine rotors as described above.
Therefore, it is clear that there is an unmet need for a low cost dual alloy radial turbine rotor that avoids fatigue in the saddle regions between blades.
There is also an unmet need for a dual alloy radial turbine rotor that has non-cooled blades and is as resistant to creep rupture failure as a cooled turbine rotor subjected to the same temperatures.